There is a delicate-regulated redox system in living organism. In a healthy human body, any harmful oxidants, whether from endogenous or exogenous origin, are efficiently neutralized by corresponding reductases in a timely manner so as to protect important biological macromolecules such as proteins, DNA and lipids, from being oxidized. However, under unbalanced conditions when there are more amounts of harmful oxidants than the capacity of reductases to deal with, which is called the state of oxidative stress, excessive oxidants can oxidize important biological macromolecules such as proteins, DNA and lipids, leading to malfunction or dysfunction of these important biological macromolecules and resulting in serous illness or even death. These harmful oxidants are basically reactive oxygen species (ROS) and reactive nitrogen species (RNS). Among them, the majority are free radicals.
There are three sources of free radicals for a human body. Internal source includes mitochondria, inflammation, exercise, xanthine oxidase, peroxisomes, phagocytes, etc. Free radicals from internal source are actually generated from normal metabolic cycles. Part of them is used by immune system to fight against the invasion of bacteria or virus. Their functions also include redox signaling, cleaning up death cells, activating and modulating some important life processes, etc. Overall, free radicals from internal source are often kept under control by the redox system. External source includes cigarette smoke, alcoholism, toxins, certain drugs, ozone, UV light, radiation, pesticides, herbicides, environmental pollutants, etc. The third source is related to physiological factors, including stress, emotion, disease conditions, etc. Free radicals from the latter two sources are usually extra burdens of the redox system and they are the roots of oxidative stress that causes health problems.
Free radicals have been implicated in the etiology of large number of major diseases, such as Alzheimer's disease, Parkinson's disease, cancers, diabetes, HIV, acne, cardiovascular disease, renal disease, hypertension, hypercholesterolemia, hyperlipidemia, rheumatoid arthritis, inflammation, pain, aging, stroke, cataract, glaucoma, age-related macular degeneration, etc. Antioxidants that can combat free radicals have drawn significant attention in past decades. Natural sources of antioxidants include fruits, vegetables and other dietary. Herbal polyphenols, flavonoids, beta-carotene, vitamin A, vitamin C, vitamin E, lipoic acid, dithiolethione, ovothiol, glutathione and melatonin are some examples of naturally occurring antioxidants. However, some problems might be encountered when naturally occurring antioxidants are directly used as drugs. Their bioavailability may not be good enough because many of them have poor solubility in water. Their antioxidant power may not precisely fit in the indication. Some polyphenols can form precipitates with proteins (enzymes) down the digestive tract, resulting in poor bioavailability and causing digestion issue.
There has been much less number of synthetic organic antioxidants so far than that of natural occurring antioxidants. Edaravone, pirenoxine, phacolin and bendazac are some examples of synthetic organic antioxidants. These four are not approved by FDA for use as drugs in US. But they are approved for use as drugs in some other countries, even though none of them has remarkable therapeutic effects nor is a drug for major diseases. BHT(2,6-Di-tert-butyl-4-methylphenol) is another example of synthetic organic antioxidant, which is widely used gas a stabilizer for storage of some organic solvents, such as THF and diethyl ether, etc., to protect them from being air-oxidized. Oltipraz, DTT (dithiothreitol), probucol and succinobucol are other examples of synthetic organic antioxidants. None of these has been approved as antioxidant drug, although oltipraz is used as a schistosomicide. But the advantages of synthetic organic antioxidants are clear that they can be designed in such a way to enhance the bioavailability, to minimize their toxicities, to tune in the scavenging power on free radicals, and so forth. Synthetic antioxidants can certainly play an important role in treatment and/or prevention of major diseases that pare associated with oxidative stress.
By definition, antioxidants are a class of compounds that can deactivate reactive oxidants by means of being oxidized themselves. As of being readily oxidized, the organic antioxidants are often aniline-like compounds (including indole-like), phenol or polyphenol compounds, thiol-containing compounds and selenol-containing compounds. In order to be used as drugs, both reduced form and oxidized form of the ideal antioxidant should not have any toxicity issues. For example, phosphine compounds are known to be easily oxidized and can be good antioxidants in chemical wise. But they are ruled out as drugs due to toxicity issues.
The theory of free radicals and antioxidants related to human health is widely accepted in main stream of science worldwide. Numerous research papers have been published in large scope of scientific journals and this trend is still continuing. Positive results are reported from many vitro and vivo tests, and even reported from some early phase clinical trials. Since 1990's, however, several strictly designed clinical trials have shown inconclusive results, no efficacy or high adverse effects on high dose of several antioxidants, casting shadows on this research area. A breakthrough is urgently needed.